1. Field of the Invention
The present invention relates to a semiconductor device having aluminum interconnection formed of an aluminum film or an aluminum-based alloy film on a surface of a semiconductor substrate.
2. Description of the Prior Art
As interconnection material for the semiconductor devices using semiconductors such as silicon (Si), usually aluminum (Al) film or aluminum alloy film which includes aluminum as a major component and includes silicon (Si), copper (Cu), palladium (Pd), titanium (Ti), scandium (Sc), boron (B), etc. is used. It has been well known that performance of such aluminum interconnection is degraded to reduce reliability due to "the electromigration phenomenon" that aluminum atoms are transported by electrons when a large current flows through the aluminum interconnection. Since such electromigration phenomenon is accelerated in compliance with increase in the current density in the wirings and increase in the temperature of the wirings, it causes a serious problem in the semiconductor devices, particularly, devices which are miniaturized like memory devices, i.e., highly integrated devices or power devices for supplying high density current.
As for an aluminum interconnection for the semiconductor device having high electromigration endurance, aluminum alloy film in which copper, etc. are added into the aluminum film and the like, are used in the prior art. Various improvements have already been proposed. Particularly, it has been known that, when orientation of the aluminum film is evaluated by X-ray diffraction, the aluminum film having high (111) orientation exhibits high electromigration endurance. That finding has been known in the literature Proc. of 29th International Reliability Physics Symposium, 1991, p.97! reported by Kageyama et al. and the literature Japanese Journal of Applied Physics, Vol.32 (1993), pp.4479-4484! by H. Shibata et al., for example. In addition, the relationship between FWHM (Full Width at Half Maximum) of the X-ray diffraction peak from aluminum (111) plane and electromigration endurance has been disclosed in the literature Atomic Transportation in LSI Interconnection, preprint of Stress Problem Society, pp.19-20, Thin Film/Surface Symposium of Institute of Japanese Applied Physics, May 27 1994, Tokyo! reported by Toyoda et al. It has been known that electromigration endurance becomes high as the aluminum interconnection has narrower FWHM of (111) peak by X-ray diffraction.
In general, if (111) surfaces formed as a closely packed surface is used as the surface and boundary surface in the face-centered cubic lattice of the crystal structure, the aluminum interconnection may take (111) orientation since energy of system can be minimized. However, in an aluminum film formed on the silicon oxide film, the extent of the (111) orientation is low and therefore FWHM of (111) peak by X-ray diffraction is as wide as several degrees. This result is due to the following causes.
More particularly, in the manufacturing steps of the semiconductor device, heating temperature of the semiconductor substrate in forming the aluminum film is relatively high, although it is lower than melting point of aluminum or aluminum alloy. For this reason, before the aluminum film is grown as a continuous film, it grows in an island-like fashion, as shown in FIG. 1, at the initial process of a film forming step for the aluminum film. At this time, the surface of the island-like aluminum film 57 forms (111) surface which is most stable in respect of energy. Between the surface of the insulating film 4 as the under-layer and the (111) surface of crystals in the island-like aluminum film 57, there exists an inclination angle determined by the wetting angle, or the contact angle between the aluminum film and the under-layer. For example, an inclination angle of several degrees is obtained if the silicon oxide film is used as the under-layer for the aluminum film. After the aluminum film has been grown as the continuous film with the progress of the aluminum film formation, the surface of the insulating film 4 and the surface of the aluminum film become parallel. However, since relationship between the surface of the insulating film 4 and the (111) surface of the aluminum film is held at the initial state of the aluminum film formation, the surface of the aluminum film and the (111) surface takes an inclination angle of several degrees. In such an aluminum film, when crystal orientation parallel to the surface is measured by X-ray diffraction, intensity for the (111) orientation is weak and electromigration endurance is low. This finding has been widely known in the report proposed by Toyoda et at., for example.
As an example of such aluminum film, aluminum (111) reflection rocking curve measured by X-ray diffraction is shown in FIG. 2 when the 50 nm thick silicon thermal oxide film is formed on the silicon substrate, and the 2 .mu.m thick Al-1% Si film are formed by DC magnetron sputtering. It can be found from the rocking curve that (111) surface of the crystals in the aluminum film is inclined relative to the surface of the aluminum film by an average of 4.5 degrees and that the FWHM of the peak of (111) reflection rocking curve is as wide as about 6.7 degrees so that crystallographic quality of the aluminum film is not always good.
As the approach to improve the degree of (111) orientation of the aluminum film, there are three known approaches, i.e., (a) The aluminum film is epitaxially grown on the titanium nitride (TiN) film which has a lattice constant close to that of the aluminum film, (b) The non-crystalline film (or the amorphous film) is used as the under-layer, and (c) The metal film which is highly reactive to Al is used as the under-layer.
These three approaches will be explained hereinbelow.
(a) It has been known in the report by Kageyama et al. to have the aluminum film epitaxially grown on the titanium nitride (TiN) film which has a lattice constant close to that of the aluminum film (first approach). This approach is intended to improve the degree of (111) orientation of the aluminum film by epitaxially growing the aluminum film on the titanium nitride (TiN) film which has (111) preferred orientation like the aluminum film and has a lattice constant (0.4239 nm) close to that (0.4049 nm) of the aluminum film.
However, the following problems reside in this first approach. First, in this approach wherein the aluminum film is epitaxially grown on the titanium nitride (TiN) film which has a lattice constant close to that of the aluminum film, there is a problem that mismatching between the lattice constant (0.4239 nm) of the titanium nitride film and the lattice constant (0.4049 nm) of the aluminum film is as large as about 4.5%. Although the film can as a rule be epitaxially grown so far as lattice mismatching between the under-layer and the film to be formed is within about 5%, the crystallographic quality and the degree of orientation of the film to be formed is degraded with increase of the lattice mismatching. Therefore, it can be concluded that to use the titanium nitride (TiN) film is not always sufficient from a view point of improving the degree of orientation of the aluminum film by virtue of epitaxial growth.
(b) Next, it has been known in the report by Toyoda et al. to use the amorphous film as the under-layer (second approach). This second approach intends to improve the degree of (111) orientation of the aluminum film by changing the growth mechanism of the aluminum film from island-like growth into layer-like growth by means of increasing surface energy of the under-layer at initial stage of the aluminum film formation.
However, a problem existing in the second approach is that it is difficult to form non-crystalline film without degrading "the degrade step coverage" of the aluminum film. More particularly, there has been known to use precipitation method, quenching method, condensation method, and others as the method of forming the non-crystalline film (amorphous film). In any method, substrate temperature must be set lower than crystallization temperature of the amorphous material during forming the amorphous under-layer film and during forming the aluminum film. This finding has been known in the article by Kinbara et al., i.e., "Generation of Amorphous Thin Film by Evaporation" Applied Physics, Vol.45, No 12, pp.1165-1171, 1976!. In general, crystallization temperature of the metal is lower than that of oxide, etc. For example, for Cr metal it is 220.degree. K. (absolute temperature), and Sn--Cu alloy has about 60.degree. K. For this reason, since the substrate has to be cooled to form the non-crystalline film water or a cooling equipment, cooling apparatus with the use of liquid nitrogen or liquid helium must be equipped. Upon forming the aluminum film on the non-crystalline film, the aluminum film must be formed at the low temperature so as not to crystallize the amorphous film. Therefore, raising the temperature of the substrate with a view to improve step coverage in the recessed portions is limited. In general, although methods of forming the film in vacuum or under low pressure such as vacuum sputtering or evaporation may be used as the method of forming the aluminum film for use in interconnection of the semiconductor device, raising the temperature of the substrate is limited with a view to improve the quality of vacuum if the non-crystalline film is used as the under-film. The second approach is not preferable in this respect.
(c) It has been proposed in Patent Application Publication (KOKAI) 5-90268 to use the metal film which is highly reactive to Al as the under-layer (third approach). This third approach can be enabled since flat growth is advantageous with respect to energy in contrast to aluminum grown at the initial stage is grown in an island-like fashion by combining with each other if the metal film which is highly reactive to Al is located in the under-layer. The aluminum film to be grown flat is formed as a uniaxial orientation film in which (111) surfaces having smallest surface energy of Al are orientated.
However, in the third approach, there is an upper limit in a thickness of the under-metal. In other words, it has been known that rugged surface appears on the under-metal film and therefore the degree of (111) orientation of the Al film formed on the under-metal film is degraded when a thickness of the under-metal film exceeds 50 nm. Such an upper limit in a thickness of the under-metal causes insufficient barrier in the contact hole if the under-metal film is used as the barrier metal between Al and silicon. In the semiconductor device in which contact holes having high aspect ratio are formed, if the upper limit in the thickness of the under-metal film on the flat portion, it is hard to form the under-metal film having a sufficient thickness on the bottom portions and side wall portions of the contact holes. In some cases, uncontinuous under-metal film is formed. Such noncontinuous under-metal film and such under-metal film having an insufficient thickness on the bottom portions and side wall portions of the contact holes is not preferable since they would cause reduction in barrier characteristic or island-like growth of aluminum at the initial stage of growth process of the aluminum film.